materials

Article

Electrochemical Corrosion Behavior of Ta2NNanoceramic Coating in Simulated Body Fluid

Jian Cheng 1, Jiang Xu 1,2,*, Lin Lin Liu 1 and Shuyun Jiang 3

1 Department of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics,29 Yudao Street, Nanjing 210016, China; chengjian@nuaa.edu.cn (J.C.); daicaiyan@gmail.com (L.L.L.)

2 School of Mechanical & Electrical Engineering, Wuhan Institute of Technology, 693 Xiongchu Avenue,Wuhan 430073, China

3 Department of Mechanical Engineering, Southeast University, 2 Si Pai Lou, Nanjing 210096, China;jiangshy@seu.edu.cn

* Correspondence: xujiang73@nuaa.edu.cn; Tel.: +86-25-5211-2626

Academic Editor: Peter J. UggowitzerReceived: 2 August 2016; Accepted: 3 September 2016; Published: 10 September 2016

Abstract: In order to improve the corrosion and wear resistance of biomedical Ti-6Al-4V implants,a Ta2N nanoceramic coating was synthesized on a Ti-6Al-4V substrate by the double glow dischargeplasma process. The Ta2N coating, composed of fine nanocrystals, with an average grain size of12.8 nm, improved the surface hardness of Ti-6Al-4V and showed good contact damage toleranceand good adhesion strength to the substrate. The corrosion resistance of the Ta2N coating in Ringer’sphysiological solution at 37 C was evaluated by different electrochemical techniques: potentiodynamicpolarization, electrochemical impedance spectroscopy (EIS), potentiostatic polarization andcapacitance measurements (Mott-Schottky approach). The evolution of the surface composition ofthe passive films at different applied potentials was determined by X-ray photoelectron spectroscopy(XPS). The results indicated that the Ta2N coating showed higher corrosion resistance than bothcommercially pure Ta and uncoated Ti-6Al-4V in this solution, because of the formed oxide filmon the Ta2N coating having a smaller carrier density (Nd) and diffusivity (Do) of point defects.The composition of the surface passive film formed on the Ta2N coating changed with the appliedpotential. At low applied potentials, the oxidation of the Ta2N coating led to the formation of tantalumoxynitride (TaOxNy) but, subsequently, the tantalum oxynitride (TaOxNy) could be chemicallyconverted to Ta2O5 at higher applied potentials.

Keywords: Ta2N; characterization; mechanical properties; polarization; electrochemical impedance;Mott-Schottky

1. Introduction

Due to the increased numbers of elderly people in today’s society as a percentage of the populationand the growth of accidents, there is ever-increasing demand for artificial joint replacements tosurgically substitute for diseased or lost bones to restore form and function [1]. Metallic materialshave been extensively used as hard tissue replacements for artificial bone and joint implants.Compared to more conventional stainless steels and cobalt-based alloys, titanium and its alloyshave greater advantages for applications in orthopedic implants, because of their lower modulus,lower density, superior biocompatibility and better corrosion resistance [2]. Among various typesof Ti alloys, Ti-6Al-4V is the first, and the most commonly used, implant material, particularlyfor orthopedic prosthesis applications. When implants are placed in the human body, they arein contact with extracellular body fluids, such as blood and interstitial fluids, which not onlycontain high concentrations of chloride ions, but also amino acids and proteins. Hence, under such

Materials 2016, 9, 772; doi:10.3390/ma9090772 www.mdpi.com/journal/materials

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an aggressive environment, the corrosion resistance of the metallic implant becomes important, sincethe corrosion rate is related to the release of metal ions into the tissue surrounding the implant, which,in turn, significantly influences biocompatibility [3]. Corrosion resistance of Ti-6Al-4V in variousenvironments is attributed to the presence of a highly stable and tenaciously adherent passive filmspontaneously formed on its surface. This plays a decisive role in determining its corrosion resistanceand biocompatibility. However, clinical experience has shown that Ti-6Al-4V is susceptible to localizedcorrosion in a biological environment, causing the release of metal ions into the tissue adjacent tothe implants. The release of metal ions is found to not only be detrimental to bone attachment andfurther bone growth on the implant surface [4,5], but also causes long-term health problems, such asperipheral neuropathy, osteomalacia and Alzheimer diseases [5,6]. In addition, for load-bearing metallicorthopaedic implants, wear occasionally occurs in cyclic load bearing areas, leading to the generation ofdebris and the degradation of the passive oxide film integrity to accelerated corrosion damage. Owing toits low hardness, the poor wear resistance of Ti-6Al-4V promotes aseptic loosening after long-termimplantation and these abraded debris particles cause inflammatory reaction in the tissue [7].

As is well known, the material surface exerts an important influence on the response of thebiological environment to artificial medical devices. Various surface modification techniques [8] havebeen explored as a means to improve the corrosion and wear resistance of titanium and its alloys,without sacrificing its excellent bulk attributes, such as relatively low modulus, good fatigue strength,formability and machinability. The most effective way to achieve this is to deposit various ceramiccoatings, e.g., transition metal oxides, nitrides, carbides, or silicides [9–11]. These can provide a barrierlayer between the bioenvironment and the alloy substrate. Recently, due to their high thermal stability,stable electrical resistivity, wear resistance and superior corrosion, tantalum nitrides have receivedmuch attention as protective coating materials and have been applied in a range of mechanical andmicroelectronic industries, including application as hard coatings for cutting tools [12], diffusionbarrier layers in integrated circuits [13], and stable thin film resistors [14]. Further, similar to metallicTa, tantalum nitride based materials (for instance, TaN, Ta2N and TaCxN1−x) have enormous potentialin biomedical fields. Leng et al. found that tantalum nitride films showed better blood compatibilitythan low-temperature isotropic pyrolytic carbon (LTIC), and suggested that they could be a possiblealternative for LTIC in artificial heart valves [15]. Nonetheless, data on the electrochemical behaviorof tantalum nitride in physiological solutions are still scarce. In the present work, a novel Ta2Nnanoceramic coating was prepared on Ti-6Al-4V substrate by a double glow discharge plasma process.The microstructure of the Ta2N nanoceramic coating was characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hardness andadhesion strength of the coating to the substrate were evaluated using nanoindentation and scratchtesting, respectively. To evaluate its potential use in biomedical applications, the electrochemicalbehavior of the Ta2N coating was examined in Ringer’s solution by various electrochemical methodsincluding potentiodynamic polarization, potentiostatic polarization, EIS and Mott-Schottky analysis.The corrosion resistance of commercially pure Ta and uncoated Ti-6Al-4V was also measured toprovide a comparative study.

2. Materials and Methods

2.1. Coating Specimen Preparation

The samples were cut from a medical grade Ti-6Al-4V alloy bar. The Ti-6Al-4V substrates wereabraded with 2000-grit SiC paper, polished with 0.5 µm Cr2O3 powder and then cleaned usingdouble-distilled water. The sputtering target is a 99.99% pure Ta disk with a diameter of 100 mm andthickness of 5 mm. The Ta2N coating was fabricated on a Ti-6Al-4V substrate using a double cathodeglow discharge apparatus. During the preparation of coating samples, one cathode served as thesputtering target, and the other cathode is the substrate material, as described in detail elsewhere [10].The base pressure in the chamber was evacuated down to 5 × 10−4 Pa. Prior to deposition, the

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substrate surface was cleaned by argon ion sputtering for 10 min to remove surface contamination atabout 20 Pa and a substrate bias voltage of −600 V. During deposition, the working pressure of 35 Pais composed of Ar and N2 gas mixture, with N2:Ar flux ratio of 1:20. The sputtering parameters ofdeposition used here include: the target electrode bias voltage of −900 V, substrate bias voltage of−350 V; substrate temperature of 850 C, a parallel distance between the target and the substrate of10 mm and a deposition time of 1.5 h.

2.2. Phase and Microstructural Characterization

The crystal structure of the as-deposited coating was analyzed by a D8 ADVANCE X-raydiffractometer using Cu Kα radiation. The surface and cross-sectional morphology of the as-preparedcoating were determined using a S-4800 field emission scanning electron microscopy. TEM observationswere performed using a JEOL JEM-2010 transmission electron microscope operating at an acceleratingvoltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a KratosAXIS Ultra ESCA system using Al Ka (1486.6 eV, pass energy of 20 eV). The deconvolution of thespectra was achieved by fitting the data using the software XPS PEAK4.1. Peak identification wasperformed with reference to an XPS database.

2.3. Mechanical Properties Measurements

Nanoindentation tests were conducted on the Ta2N coating and Ti-6Al-4V substrate usinga nanoindentation tester (Ultra-Micro Indentation System 2000), equipped with a Berkovich diamondtip. The standard analysis procedure proposed by Oliver and Pharr [16] was used to determine thehardness of the samples from the load–displacement curves. The adhesion strength of the Ta2Ncoating to Ti-6Al-4V substrate was evaluated by a WS-2005 scratch tester equipped with an acousticemission (AE) detector. During the scratch tests, a 200 µm radius Rockwell C diamond indenter wasdrawn across the coating surface under a linearly increased normal load from 0 N to 100 N witha constant loading rate of 20 N/min. The contact-damage resistance of the coating was assessed usingan applied normal load of 9.80 N with a Vickers indenter. A focused ion beam (FIB) milling system(Nova Nanolab 200, FEI, Hillsboro, OR, USA) was used to obtain subsurface sectioning and imagingof indentation sites.

2.4. Electrochemical Measurements

The electrochemical measurements were conducted using a CHI604D electrochemical workstation(Shanghai ChenHua Instruments Co., Shanghai, China) in a three-electrode configuration. The testspecimen, a platinum plate and a saturated calomel electrode (SCE) were used as the working, auxiliary,and reference electrodes, respectively. The work electrolyte was Ringer’s physiological solution, whichcontains 8.61 g of NaCl, 0.49 g of CaCl2 and 0.30 g of KCl diluted in 1 L of water. Ringer’s solutionwas naturally aerated and maintained at 37 ± 0.5 C to simulate the body temperature. Prior toelectrochemical experiments, working electrodes were initially reduced potentiostatically at −0.8 Vfor 10 min to remove air-formed oxides. The polarization curves were recorded potentiodynamicallyat a scan rate of 20 V/min from −0.5 to 1.5 V. Electrochemical impedance spectroscopy (EIS)measurements were conducted over the frequency from 100 kHz to 10 mHz with a 10 mV amplitudesignal at open circuit potential (EOCP). ZSimpWin software was used to fit the appropriate electricalequivalent circuit (EEC) model into the EIS experimental data. In order to characterize the electronicproperties of the passive films, steady-state passive films were formed by potentiostatically polarizingthe specimens at different film formation potentials (Ef, 0.4 V, 0.6 V, 0.8 V and 1.0 V) for 60 min.The Mott-Schottky analysis were performed by sweeping the potential from Ef in the negativedirection with potential steps of 25 mV, at a frequency of 1 kHz with an amplitude signal of 10 mV.Uncoated Ti-6Al-4V and commercially pure Ta (99.99% pure) were used as reference materials for theelectrochemical measurements.

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3. Results and Discussion

3.1. Microstructure and Phase Analysis

A typical X-ray diffraction pattern recorded from the Ta2N coating is shown in Figure 1. All thediffraction peaks acquired from the as-deposited coating can be indexed to the hexagonal structuredTa2N phase based on data in JCPDS card No. 26-0985. No other characteristic peaks were detected,indicating that the coating consists of single phase Ta2N. In addition, clear diffraction peak broadeningwith relatively low peak intensity can be observed in the pattern, suggesting that the coating containsvery fine grains. Scherrer analysis of the X-ray diffraction pattern give an average crystal size of~10.5 nm from the (101) plane, after correction for instrumental line broadening. Figure 2a,b show,respectively, representative plan view and cross-sectional SEM images for the Ta2N coated Ti-6Al-4V.As shown in Figure 2a, the Ta2N coating has a relatively smooth and uniform surface. The SEMcross-sectional view (Figure 2b) of the Ta2N coated Ti-6Al-4V reveals that the Ta2N coating, withan average thickness of ~25 µm, is of good quality without any visible defects and tightly adhered tothe Ti-6Al-4V substrate.

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3. Results and Discussion

3.1. Microstructure and Phase Analysis

A typical X-ray diffraction pattern recorded from the Ta2N coating is shown in Figure 1. All the diffraction peaks acquired from the as-deposited coating can be indexed to the hexagonal structured Ta2N phase based on data in JCPDS card No. 26-0985. No other characteristic peaks were detected, indicating that the coating consists of single phase Ta2N. In addition, clear diffraction peak broadening with relatively low peak intensity can be observed in the pattern, suggesting that the coating contains very fine grains. Scherrer analysis of the X-ray diffraction pattern give an average crystal size of ~10.5 nm from the (101) plane, after correction for instrumental line broadening. Figure 2a,b show, respectively, representative plan view and cross-sectional SEM images for the Ta2N coated Ti-6Al-4V. As shown in Figure 2a, the Ta2N coating has a relatively smooth and uniform surface. The SEM cross-sectional view (Figure 2b) of the Ta2N coated Ti-6Al-4V reveals that the Ta2N coating, with an average thickness of ~25 μm, is of good quality without any visible defects and tightly adhered to the Ti-6Al-4V substrate.

Figure 1. Typical X-ray diffraction pattern taken from the as-deposited Ta2N coating.

Figure 2. (a) Plan view and (b) cross-sectional SEM morphologies of the Ta2N coating.

Figure 1. Typical X-ray diffraction pattern taken from the as-deposited Ta2N coating.

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3. Results and Discussion

3.1. Microstructure and Phase Analysis

A typical X-ray diffraction pattern recorded from the Ta2N coating is shown in Figure 1. All the diffraction peaks acquired from the as-deposited coating can be indexed to the hexagonal structured Ta2N phase based on data in JCPDS card No. 26-0985. No other characteristic peaks were detected, indicating that the coating consists of single phase Ta2N. In addition, clear diffraction peak broadening with relatively low peak intensity can be observed in the pattern, suggesting that the coating contains very fine grains. Scherrer analysis of the X-ray diffraction pattern give an average crystal size of ~10.5 nm from the (101) plane, after correction for instrumental line broadening. Figure 2a,b show, respectively, representative plan view and cross-sectional SEM images for the Ta2N coated Ti-6Al-4V. As shown in Figure 2a, the Ta2N coating has a relatively smooth and uniform surface. The SEM cross-sectional view (Figure 2b) of the Ta2N coated Ti-6Al-4V reveals that the Ta2N coating, with an average thickness of ~25 μm, is of good quality without any visible defects and tightly adhered to the Ti-6Al-4V substrate.

Figure 1. Typical X-ray diffraction pattern taken from the as-deposited Ta2N coating.

Figure 2. (a) Plan view and (b) cross-sectional SEM morphologies of the Ta2N coating. Figure 2. (a) Plan view and (b) cross-sectional SEM morphologies of the Ta2N coating.

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A bright-field/dark-field pair, Figure 3a,b, show clearly that the microstructure of the coating iscomposed of uniformly distributed fine nanocrystals with irregular morphologies. The Debye-Scherrerrings in the selected area diffraction (SAD) pattern (Figure 3c) from the inside to outside are assignedto the hexagonal structured Ta2N (100), (101), (102), (110), (103), (112) and (211) planes, respectively,which is in complete agreement with the XRD results. Figure 3d shows the grain size distributionbased on the analysis of more than 120 grains (Figure 3d). The results indicate that the majority of thegrains have a grain size smaller than 15 nm and the average grain size based on statistical analysis is12.8 nm. It is worthwhile to note that the average grain size estimated by TEM is slightly larger thanthe grain size measured by XRD, because the XRD data neglect contributions from structural defects(i.e., stacking faults or low-angle grain boundaries) and microstraining [17]. From the bright-fieldhigh-resolution TEM (HRTEM) image shown in Figure 3e, the nanocrystallites have clear lattice fringesand interplanar distance of the crystallites marked with a white frame was calculated to be ~0.235 nm,corresponding to the d-spacing of the 101 planes of hexagonal Ta2N.

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A bright-field/dark-field pair, Figure 3a,b, show clearly that the microstructure of the coating is composed of uniformly distributed fine nanocrystals with irregular morphologies. The Debye-Scherrer rings in the selected area diffraction (SAD) pattern (Figure 3c) from the inside to outside are assigned to the hexagonal structured Ta2N (100), (101), (102), (110), (103), (112) and (211) planes, respectively, which is in complete agreement with the XRD results. Figure 3d shows the grain size distribution based on the analysis of more than 120 grains (Figure 3d). The results indicate that the majority of the grains have a grain size smaller than 15 nm and the average grain size based on statistical analysis is 12.8 nm. It is worthwhile to note that the average grain size estimated by TEM is slightly larger than the grain size measured by XRD, because the XRD data neglect contributions from structural defects (i.e., stacking faults or low-angle grain boundaries) and microstraining [17]. From the bright-field high-resolution TEM (HRTEM) image shown in Figure 3e, the nanocrystallites have clear lattice fringes and interplanar distance of the crystallites marked with a white frame was calculated to be ~0.235 nm, corresponding to the d-spacing of the 101 planes of hexagonal Ta2N.

Figure 3. (a) Bright-field and (b) dark-field plan-view TEM images; (c) corresponding selected area electron diffraction (SAED) pattern; (d) statistical histogram of the Ta2N grain sizes and (e) high resolution TEM image for the as-deposited Ta2N coating.

Figure 3. (a) Bright-field and (b) dark-field plan-view TEM images; (c) corresponding selected areaelectron diffraction (SAED) pattern; (d) statistical histogram of the Ta2N grain sizes and (e) highresolution TEM image for the as-deposited Ta2N coating.

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3.2. Mechanical Properties

Typical load-displacement curves for the performed nano-indentation tests for both the Ta2Ncoating and uncoated Ti-6Al-4V are plotted in Figure 4a. The curve for the Ta2N coating is continuous,indicating no occurrence of any abrupt cracking of the coating. Under the same loading conditions, theresidual indentation depth and maximum indentation depth for the Ta2N coating are shallower thanthose for uncoated Ti-6Al-4V, denoting that the Ta2N coating possesses higher resistance to local plasticdeformation. The elastic recovery (ER) is defined as ER = (dmax − dres)/dmax, where dmax and dres

are the displacement at the maximum load and residual displacement after unloading, respectively.It can be found that the elastic recovery values increase from 28% for uncoated Ti-6Al-4V to 55% forthe Ta2N coating, suggesting better elastic resistance for the Ti-6Al-4V substrate coated with Ta2N.As shown in Figure 4b, the hardness and elastic modulus of the Ta2N coating were determined to be34.1 ± 0.9 GPa and 298.2 ± 6.6 GPa, respectively, which are approximately equivalent to the reportedvalues for a Ta2N coating prepared by radio-frequency (RF) magnetron sputtering [18]. Additionally,the ratio of the maximum indentation depth to the coating thickness is less than 10%, suggesting thatthe contribution to mechanical properties from the substrate can be negligible.

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3.2. Mechanical Properties

Typical load-displacement curves for the performed nano-indentation tests for both the Ta2N coating and uncoated Ti-6Al-4V are plotted in Figure 4a. The curve for the Ta2N coating is continuous, indicating no occurrence of any abrupt cracking of the coating. Under the same loading conditions, the residual indentation depth and maximum indentation depth for the Ta2N coating are shallower than those for uncoated Ti-6Al-4V, denoting that the Ta2N coating possesses higher resistance to local plastic deformation. The elastic recovery (ER) is defined as ER = (dmax − dres)/dmax, where dmax and dres are the displacement at the maximum load and residual displacement after unloading, respectively. It can be found that the elastic recovery values increase from 28% for uncoated Ti-6Al-4V to 55% for the Ta2N coating, suggesting better elastic resistance for the Ti-6Al-4V substrate coated with Ta2N. As shown in Figure 4b, the hardness and elastic modulus of the Ta2N coating were determined to be 34.1 ± 0.9 GPa and 298.2 ± 6.6 GPa, respectively, which are approximately equivalent to the reported values for a Ta2N coating prepared by radio-frequency (RF) magnetron sputtering [18]. Additionally, the ratio of the maximum indentation depth to the coating thickness is less than 10%, suggesting that the contribution to mechanical properties from the substrate can be negligible.

Figure 4. (a) Load-displacement curves and (b) hardness and elastic modulus for the Ta2N coating and uncoated Ti-6Al-4V.

The contact damage resistance of the Ta2N coating was further evaluated using the Vickers indentation method under various loads. Figure 5a shows optical micrographs of Vickers indentations in the Ta2N coating. Careful observation of the Vickers indentations, obtained at loads ranging from 0.49 to 9.80 N, reveals that indentations obtained at these various loads are nearly ‘perfect’, with no evidence of microcracking at the corners of the indent, where the coating undergoes the highest tensile stress. Similarly, no additional cracking is visible within cross-sections

Figure 4. (a) Load-displacement curves and (b) hardness and elastic modulus for the Ta2N coating anduncoated Ti-6Al-4V.

The contact damage resistance of the Ta2N coating was further evaluated using the Vickersindentation method under various loads. Figure 5a shows optical micrographs of Vickers indentationsin the Ta2N coating. Careful observation of the Vickers indentations, obtained at loads ranging from0.49 to 9.80 N, reveals that indentations obtained at these various loads are nearly ‘perfect’, with noevidence of microcracking at the corners of the indent, where the coating undergoes the highest tensilestress. Similarly, no additional cracking is visible within cross-sections taken from the plastically

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deformed region below the indent obtained at a load of 9.80 N (Figure 5c). The above results suggestthat the Ta2N coating exhibits good contact damage tolerance.

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taken from the plastically deformed region below the indent obtained at a load of 9.80 N (Figure 5c). The above results suggest that the Ta2N coating exhibits good contact damage tolerance.

Figure 5. (a) Optical micrographs of Vickers indentations in the Ta2N coating at applied loads ranging from 0.49 to 9.80 N; (b) Plan view and (c) cross-sectional secondary electron FIB images of Vickers indentation under an indentation load of 9.80 N.

The adhesion strength of a coating on a metallic implant is one of the most important properties for orthopedic applications, since the interfacial adhesive strength of the coating affects the lifetime of a biomaterial [19]. The scratch test is commonly performed to evaluate the adhesion strength of a coating/substrate. Figure 6a shows the acoustic emission signals measured as a function of normal load. As shown in Figure 6a, once the normal load has reached a critical value of ~56 N, continuous acoustic emission peaks were generated. From the scratch track evident in the SEM image (Figure 6b), both the width of the scratch track and the extent of the surface damage increase with increasing normal load, and when the normal load reaches to the critical value, a large area of delamination is evident at the edge of the scratch track. As a general rule, a critical load of above 30 N measured with a Rockwell C diamond tip during scratch testing is believed to be sufficient for most engineering applications [20]. Therefore, the Ta2N coating possesses sufficient adhesion strength to meet long-term use in the human body under load-bearing conditions.

Figure 6. (a) Acoustic emission signal peaks versus normal load curve and (b) SEM image of the scratch track for the Ta2N coating.

3.3. Open Circuit Potential Measurements

Figure 7 presents the variation of the open circuit potential (EOCP) for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V as a function of immersion time in Ringer’s

Figure 5. (a) Optical micrographs of Vickers indentations in the Ta2N coating at applied loads rangingfrom 0.49 to 9.80 N; (b) Plan view and (c) cross-sectional secondary electron FIB images of Vickersindentation under an indentation load of 9.80 N.

The adhesion strength of a coating on a metallic implant is one of the most important propertiesfor orthopedic applications, since the interfacial adhesive strength of the coating affects the lifetimeof a biomaterial [19]. The scratch test is commonly performed to evaluate the adhesion strength ofa coating/substrate. Figure 6a shows the acoustic emission signals measured as a function of normalload. As shown in Figure 6a, once the normal load has reached a critical value of ~56 N, continuousacoustic emission peaks were generated. From the scratch track evident in the SEM image (Figure 6b),both the width of the scratch track and the extent of the surface damage increase with increasingnormal load, and when the normal load reaches to the critical value, a large area of delamination isevident at the edge of the scratch track. As a general rule, a critical load of above 30 N measuredwith a Rockwell C diamond tip during scratch testing is believed to be sufficient for most engineeringapplications [20]. Therefore, the Ta2N coating possesses sufficient adhesion strength to meet long-termuse in the human body under load-bearing conditions.

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taken from the plastically deformed region below the indent obtained at a load of 9.80 N (Figure 5c). The above results suggest that the Ta2N coating exhibits good contact damage tolerance.

Figure 5. (a) Optical micrographs of Vickers indentations in the Ta2N coating at applied loads ranging from 0.49 to 9.80 N; (b) Plan view and (c) cross-sectional secondary electron FIB images of Vickers indentation under an indentation load of 9.80 N.

The adhesion strength of a coating on a metallic implant is one of the most important properties for orthopedic applications, since the interfacial adhesive strength of the coating affects the lifetime of a biomaterial [19]. The scratch test is commonly performed to evaluate the adhesion strength of a coating/substrate. Figure 6a shows the acoustic emission signals measured as a function of normal load. As shown in Figure 6a, once the normal load has reached a critical value of ~56 N, continuous acoustic emission peaks were generated. From the scratch track evident in the SEM image (Figure 6b), both the width of the scratch track and the extent of the surface damage increase with increasing normal load, and when the normal load reaches to the critical value, a large area of delamination is evident at the edge of the scratch track. As a general rule, a critical load of above 30 N measured with a Rockwell C diamond tip during scratch testing is believed to be sufficient for most engineering applications [20]. Therefore, the Ta2N coating possesses sufficient adhesion strength to meet long-term use in the human body under load-bearing conditions.

Figure 6. (a) Acoustic emission signal peaks versus normal load curve and (b) SEM image of the scratch track for the Ta2N coating.

3.3. Open Circuit Potential Measurements

Figure 7 presents the variation of the open circuit potential (EOCP) for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V as a function of immersion time in Ringer’s

Figure 6. (a) Acoustic emission signal peaks versus normal load curve and (b) SEM image of the scratchtrack for the Ta2N coating.

3.3. Open Circuit Potential Measurements

Figure 7 presents the variation of the open circuit potential (EOCP) for the Ta2N coating,commercially pure Ta and uncoated Ti-6Al-4V as a function of immersion time in Ringer’s physiological

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solution at 37 C. As can be seen, similar EOCP evolution behavior is observed for all specimens.EOCP shifts quickly towards more positive potentials during the first 10~15 min. After that, EOCP

changes more slowly until it reaches a quasi-stationary value. Such EOCP evolution behaviorsuggests that all specimens undergo spontaneous passivation due to spontaneously formed oxide filmpassivating the specimen surface in the Ringer’s physiological solution, which is typical of passivatedmaterials. Figure 7 also shows a significant ennoblement in the EOCP of the Ta2N coating comparedwith that of uncoated Ti-6Al-4V, indicating that the Ta2N coating has higher thermodynamic stableoxide film.

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physiological solution at 37 °C. As can be seen, similar EOCP evolution behavior is observed for all specimens. EOCP shifts quickly towards more positive potentials during the first 10~15 min. After that, EOCP changes more slowly until it reaches a quasi-stationary value. Such EOCP evolution behavior suggests that all specimens undergo spontaneous passivation due to spontaneously formed oxide film passivating the specimen surface in the Ringer’s physiological solution, which is typical of passivated materials. Figure 7 also shows a significant ennoblement in the EOCP of the Ta2N coating compared with that of uncoated Ti-6Al-4V, indicating that the Ta2N coating has higher thermodynamic stable oxide film.

Figure 7. Variation of open circuit potential with time for uncoated Ti-6Al-4V, commercially pure Ta and the Ta2N coating in Ringer’s physiological solution at 37 °C.

3.4. Potentiodynamic Polarization Tests

Figure 8 shows typical potentiodynamic polarization plots recorded from the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V exposed to naturally aerated Ringer’s physiological solution at 37 °C. The corresponding electrochemical parameters derived from the polarization curves are summarized in Table 1. It can be seen from Figure 8 that the shape of the polarization curves is quite similar for the three tested samples. The cathodic Tafel slopes (-βc) of the tested specimens exhibit values in the range from 116.48 to 120.63 mV/decade, which indicates that the reduction of dissolved oxygen and water takes places on the samples’ surfaces, due to fact that the measurements were carried out in the aerated condition [21]. From the inspection of the anodic branch of the polarization plots, all of the tested samples show wide passive region, in where the passive current densities are almost independent of any increase in potential. The larger anodic Tafel slopes (βa) value in comparison with the βc value indicates anodic control in the corrosion process of the three tested samples [22], implying that the existence of the protective oxide films gives rise to a typical passive character of the tested samples with a low corrosion current density. The corrosion potential for the Ta2N coating was found to be nobler than for both commercially pure Ta and uncoated Ti-6Al-4V, signifying that the Ta2N coating possess a higher electrochemical inertness in such an environment. Generally, the corrosion current density (icorr) is an important parameter to evaluate the kinetics of corrosion reactions and corrosion rate is proportional to the corrosion current density measured via polarization [23]. In other words, the lower the icorr, the lower the corrosion rate. For the Ta2N coating, the corrosion current density is very low, of the order of magnitude of about 10−9 A·cm−2, which is two orders of magnitude less than those of the two reference samples. Similarly, the current density within the passive range for a potential of 0.6 V is 4.55 × 10−8 A·cm−2 for the Ta2N coating, which is also two orders of magnitude smaller than for uncoated Ti-6Al-4V and commercially pure Ta, suggesting that the passive film formed on the Ta2N coating has higher electrochemical stability. Furthermore, defects (i.e., pores and/or pinholes) present in coatings are assumed to connect the bulk electrolyte to the substrate surface and may result in severe localized corrosion due to the galvanic effect when the coating is anodic with respect to the substrate. Assuming that the Ta2N coating is electrochemically inert at low anodic

Figure 7. Variation of open circuit potential with time for uncoated Ti-6Al-4V, commercially pure Taand the Ta2N coating in Ringer’s physiological solution at 37 C.

3.4. Potentiodynamic Polarization Tests

Figure 8 shows typical potentiodynamic polarization plots recorded from the Ta2N coating,commercially pure Ta and uncoated Ti-6Al-4V exposed to naturally aerated Ringer’s physiologicalsolution at 37 C. The corresponding electrochemical parameters derived from the polarization curvesare summarized in Table 1. It can be seen from Figure 8 that the shape of the polarization curves isquite similar for the three tested samples. The cathodic Tafel slopes (-βc) of the tested specimens exhibitvalues in the range from 116.48 to 120.63 mV/decade, which indicates that the reduction of dissolvedoxygen and water takes places on the samples’ surfaces, due to fact that the measurements were carriedout in the aerated condition [21]. From the inspection of the anodic branch of the polarization plots,all of the tested samples show wide passive region, in where the passive current densities are almostindependent of any increase in potential. The larger anodic Tafel slopes (βa) value in comparison withthe βc value indicates anodic control in the corrosion process of the three tested samples [22], implyingthat the existence of the protective oxide films gives rise to a typical passive character of the testedsamples with a low corrosion current density. The corrosion potential for the Ta2N coating was found tobe nobler than for both commercially pure Ta and uncoated Ti-6Al-4V, signifying that the Ta2N coatingpossess a higher electrochemical inertness in such an environment. Generally, the corrosion currentdensity (icorr) is an important parameter to evaluate the kinetics of corrosion reactions and corrosionrate is proportional to the corrosion current density measured via polarization [23]. In other words, thelower the icorr, the lower the corrosion rate. For the Ta2N coating, the corrosion current density is verylow, of the order of magnitude of about 10−9 A·cm−2, which is two orders of magnitude less than thoseof the two reference samples. Similarly, the current density within the passive range for a potential of0.6 V is 4.55 × 10−8 A·cm−2 for the Ta2N coating, which is also two orders of magnitude smaller thanfor uncoated Ti-6Al-4V and commercially pure Ta, suggesting that the passive film formed on the Ta2Ncoating has higher electrochemical stability. Furthermore, defects (i.e., pores and/or pinholes) presentin coatings are assumed to connect the bulk electrolyte to the substrate surface and may result in severelocalized corrosion due to the galvanic effect when the coating is anodic with respect to the substrate.

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Assuming that the Ta2N coating is electrochemically inert at low anodic overpotentials, the porosity ofthe coating (P) can be estimated from the polarization resistance, Rp, using Equation (1) [24]:

P =Rps

Rpc× 10−|4Ecorr |/βa (1)

where Rps and Rpc are the measured polarization resistance of substrate and coating, respectively,∆Ecorr is the difference of the corrosion potential between substrate and coating, βa is the anodic Tafelslope for the substrate. The polarization resistance is obtained from the potentiodynamic polarizationcurves using Equation (2) [25]:

Rp =βaβc

2.303(βa + βc)icorr(2)

where βa and βc are, respectively, the anodic and cathodic Tafel slopes, and icorr is the corrosion currentdensity (A·cm−2). The calculated porosity value of the Ta2N coating is 0.22%, confirming that thenumber of defects in the coating is low. Low porosity may be related to its thickness, since porosity ofa coating is inversely proportional to coating thickness.

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overpotentials, the porosity of the coating (P) can be estimated from the polarization resistance, Rp, using Equation (1) [24]: P = × 10 | |/ (1)

where Rps and Rpc are the measured polarization resistance of substrate and coating, respectively, ∆Ecorr is the difference of the corrosion potential between substrate and coating, βa is the anodic Tafel slope for the substrate. The polarization resistance is obtained from the potentiodynamic polarization curves using Equation (2) [25]: = 2.303( + ) (2)

where βa and βc are, respectively, the anodic and cathodic Tafel slopes, and icorr is the corrosion current density (A·cm−2). The calculated porosity value of the Ta2N coating is 0.22%, confirming that the number of defects in the coating is low. Low porosity may be related to its thickness, since porosity of a coating is inversely proportional to coating thickness.

Figure 8. Potentiodynamic polarization curves of uncoated Ti-6Al-4V, commercially pure Ta and the Ta2N coating in Ringer’s physiological solution at 37 °C.

Table 1. Electrochemical parameters extracted from potentiodynamic curves of investigated specimens in Ringer’s physiological solution at 37 °C.

Samples Ti-6Al-4V Pure Ta Ta2N Coating Ecorr (V vs. SCE) −0.24 −0.21 −0.12 βa (mV/decade) 158.08 249.25 305.16 -βc (mV/decade) 116.48 118.04 120.63

icorr (A·cm−2) 4.20 × 10−7 2.99 × 10−7 6.76 × 10−9 ipass 1 (A·cm−2) 9.86 × 10−6 3.63 × 10−6 4.55 × 10−8

Rp (Ω·cm2) 6.93 × 104 1.16 × 105 5.55 × 106 p (%) – – 0.22

1 The passive current densities were derived at 0.6 V vs. SCE.

3.5. Electrochemical Impedance Spectroscopy (EIS) Measurements

The impedance spectra recorded at the open-circuit potentials (OCPs) for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V in naturally aerated Ringer’s physiological solution at 37 °C are displayed in the form of both Nyquist plots, and Bode amplitude and phase angle plots in Figure 9. The Nyquist plots for the tested samples show similar features, i.e., the response of the Nyquist complex plane are characterized by a unique capacitive loop with a wide incomplete semicircle over the whole range of frequency, typical behavior for passive materials [26]. It is

Figure 8. Potentiodynamic polarization curves of uncoated Ti-6Al-4V, commercially pure Ta and theTa2N coating in Ringer’s physiological solution at 37 C.

Table 1. Electrochemical parameters extracted from potentiodynamic curves of investigated specimensin Ringer’s physiological solution at 37 C.

Samples Ti-6Al-4V Pure Ta Ta2N Coating

Ecorr (V vs. SCE) −0.24 −0.21 −0.12βa (mV/decade) 158.08 249.25 305.16-βc (mV/decade) 116.48 118.04 120.63

icorr (A·cm−2) 4.20 × 10−7 2.99 × 10−7 6.76 × 10−9

ipass1 (A·cm−2) 9.86 × 10−6 3.63 × 10−6 4.55 × 10−8

Rp (Ω·cm2) 6.93 × 104 1.16 × 105 5.55 × 106

p (%) – – 0.221 The passive current densities were derived at 0.6 V vs. SCE.

3.5. Electrochemical Impedance Spectroscopy (EIS) Measurements

The impedance spectra recorded at the open-circuit potentials (OCPs) for the Ta2N coating,commercially pure Ta and uncoated Ti-6Al-4V in naturally aerated Ringer’s physiological solutionat 37 C are displayed in the form of both Nyquist plots, and Bode amplitude and phase angle plotsin Figure 9. The Nyquist plots for the tested samples show similar features, i.e., the response of theNyquist complex plane are characterized by a unique capacitive loop with a wide incomplete semicircle

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over the whole range of frequency, typical behavior for passive materials [26]. It is generally acceptedthat the larger the diameter of the semicircle, the better the corrosion resistance of the sample. It isevident from Figure 9a that the diameter of the capacitive semicircle of the Ta2N coating is markedlylarger than those of the two reference samples, indicating a higher corrosion resistance for the coating.In addition, the Zimag/Zre ratio of the Ta2N coating is also greater than those of the two referencesamples, indicating an enhanced capacitive behavior for the solid/liquid interface. As shown inFigure 9b, the corresponding Bode-phase plots for the tested specimens are similar in shape with highsymmetry. In the high frequency region, the log|Z| is almost frequency-independent with the phaseangle falling rapidly towards zero with increasing frequency, indicative of a representative response ofthe electrolyte resistance. In the intermediate frequency region, a linear relationship between the log|Z| and log f is observed with a slope approaching −1 and a phase angle close to −90, especiallyin the case of the Ta2N coating. This is a typical capacitive response, associated with the presenceof passive films on the tested samples acting as an efficient barrier to corrosion attack. In the lowfrequency region, the phase angles for the tested samples shift to the lower values suggesting thedecreased capacitive influence in the electrochemical behavior of the samples. Both the impedancemodulus at the lowest frequency (|Z|f→0) and the phase angle maximum for the Ta2N coating arebigger than those of the two reference samples, indicating better protective properties of the oxide filmformed on the coating.

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generally accepted that the larger the diameter of the semicircle, the better the corrosion resistance of the sample. It is evident from Figure 9a that the diameter of the capacitive semicircle of the Ta2N coating is markedly larger than those of the two reference samples, indicating a higher corrosion resistance for the coating. In addition, the Zimag/Zre ratio of the Ta2N coating is also greater than those of the two reference samples, indicating an enhanced capacitive behavior for the solid/liquid interface. As shown in Figure 9b, the corresponding Bode-phase plots for the tested specimens are similar in shape with high symmetry. In the high frequency region, the log|Z| is almost frequency-independent with the phase angle falling rapidly towards zero with increasing frequency, indicative of a representative response of the electrolyte resistance. In the intermediate frequency region, a linear relationship between the log |Z| and log f is observed with a slope approaching −1 and a phase angle close to −90°, especially in the case of the Ta2N coating. This is a typical capacitive response, associated with the presence of passive films on the tested samples acting as an efficient barrier to corrosion attack. In the low frequency region, the phase angles for the tested samples shift to the lower values suggesting the decreased capacitive influence in the electrochemical behavior of the samples. Both the impedance modulus at the lowest frequency (|Z|f→0) and the phase angle maximum for the Ta2N coating are bigger than those of the two reference samples, indicating better protective properties of the oxide film formed on the coating.

Figure 9. (a) Nyquist and (b) Bode plots of uncoated Ti-6Al-4V, commercially pure Ta and the Ta2N coating at respective open circuit potentials in Ringer’s physiological solution at 37 °C. Symbols are experimental data and solid lines are fitted results.

To gain better insight regarding the corrosion occurring on the samples, EIS spectra were analyzed using equivalent electric circuits shown in Figure 10. As shown in Figure 9, the impedance spectra of the tested samples exhibit only one time constant, indicating that the sample-electrolyte interface could be modeled by a resistance Rp and a constant phase element (CPE, Qp) in parallel, which is connected in series with the solution resistance (Rs). In this circuit, a constant phase

Figure 9. (a) Nyquist and (b) Bode plots of uncoated Ti-6Al-4V, commercially pure Ta and the Ta2Ncoating at respective open circuit potentials in Ringer’s physiological solution at 37 C. Symbols areexperimental data and solid lines are fitted results.

To gain better insight regarding the corrosion occurring on the samples, EIS spectra were analyzedusing equivalent electric circuits shown in Figure 10. As shown in Figure 9, the impedance spectra ofthe tested samples exhibit only one time constant, indicating that the sample-electrolyte interface could

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be modeled by a resistance Rp and a constant phase element (CPE, Qp) in parallel, which is connectedin series with the solution resistance (Rs). In this circuit, a constant phase element (CPE, Qp), instead ofpure capacitance, is used to obtain a satisfactory fit. The physical origin of the CPE behavior has beenwidely discussed in the literature, and it is generally believed to originate from current and potentialheterogeneous distributions due to surface disorder and roughness [27]. The impedance of ZCPE isdefined as ZCPE = [Q(jω)n]−1, where Q is the CPE constant (Ω−1·cm−2·sn), ω is the angular frequency(rad·s−1), j2 = −1 is an imaginary number, and n is the CPE exponent. Using the proposed equivalentcircuits, a good match between the measured and simulated data was achieved for each spectrum.The typical chi-square (χ2) values varied from 5.61 × 10−4 to 9.35 × 10−4, indicating a satisfactory fit.Table 2 compiles numerical values of the circuit elements from the fitting procedure. From Table 2, theresistance value (Rp) of the Ta2N coating is three times greater than that of commercially pure Ta andone order of magnitude higher than that of uncoated Ti-6Al-4V. The resistance (R) values were highlydependent on the solution in which the measurements were made, whereas the capacitance (C) valuesare independent of the solution used and thus can provide information about the dielectric property ofthe passive film [28]. The effective capacitance (Cp) can be calculated from the proposed equivalentcircuit model with a CPE using the expression developed by Brug et al. [29]

Cp = Qp1/n(Rs

−1 + Rp−1)

(n−1)/n(3)

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element (CPE, Qp), instead of pure capacitance, is used to obtain a satisfactory fit. The physical origin of the CPE behavior has been widely discussed in the literature, and it is generally believed to originate from current and potential heterogeneous distributions due to surface disorder and roughness [27]. The impedance of ZCPE is defined as ZCPE = [Q(jω)n]−1, where Q is the CPE constant (Ω−1·cm−2·sn), ω is the angular frequency (rad·s−1), j2 = −1 is an imaginary number, and n is the CPE exponent. Using the proposed equivalent circuits, a good match between the measured and simulated data was achieved for each spectrum. The typical chi-square (χ2) values varied from 5.61 × 10−4 to 9.35 × 10−4, indicating a satisfactory fit. Table 2 compiles numerical values of the circuit elements from the fitting procedure. From Table 2, the resistance value (Rp) of the Ta2N coating is three times greater than that of commercially pure Ta and one order of magnitude higher than that of uncoated Ti-6Al-4V. The resistance (R) values were highly dependent on the solution in which the measurements were made, whereas the capacitance (C) values are independent of the solution used and thus can provide information about the dielectric property of the passive film [28]. The effective capacitance (Cp) can be calculated from the proposed equivalent circuit model with a CPE using the expression developed by Brug et al. [29] = / ( + )( )/ (3)

Figure 10. Electronic equivalent circuit (EEC) used in the fitting procedure of the EIS experimental data.

Table 2. Electrochemical parameters derived from impedance fitting for investigated specimens at their respective open circuit potentials in Ringer’s physiological solution at 37 °C.

Samples Ti-6Al-4V Pure Ta Ta2N Coating Rs (Ω·cm2) 26.25 ± 0.17 25.21 ± 0.20 34.74 ± 0.35

Qp (Ω−1·cm−2·sn) (1.15 ± 0.01) × 10−5 (1.26 ± 0.01) × 10−5 (5.72 ± 0.05) × 10−6 n 0.911 ± 0.001 0.919 ± 0.001 0.941 ± 0.002

Rp (Ω·cm2) (5.75 ± 0.15) × 105 (1.21 ± 0.04) × 106 (4.70 ± 0.31) × 106 Cp (μF·cm−2) 8.43 6.19 3.35

τ (s) 4.85 7.49 15.75 χ2 9.35 × 10−4 5.61 × 10−4 6.33 × 10−4

The Cp values for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V are 3.35, 6.19 and 8.43 μF·cm−2, respectively. Higher resistance, combined with lower capacitance, endows the Ta2N coating with higher insulating or dielectric property as compared to commercially pure Ta and uncoated Ti-6Al-4V. This is consistent with the result of the polarization tests. In addition, the n value of the Ta2N coating is greater than those of both commercially pure Ta and uncoated Ti-6Al-4V, reflecting that the passive film formed on the Ta2N coating is more homogeneous and compact than those formed on the two reference samples [30]. The time constant (τ) describing the charge transport process in the passive layer can be expressed as τ = C × R. This parameter can be directly used to evaluate the rate of relevant electrochemical process, including ionic migration within the oxide film and charge transfer [31,32]. According to the results shown in Table 2, τ values of the tested samples increase in the order of uncoated Ti-6Al-4V < commercially pure Ta < the Ta2N coating. About a three times enhancement in time constant is achieved by implementation of a Ta2N coating on the Ti-6Al-4V substrate, indicating that the Ta2N coating can effectively retard the electrochemical corrosion process.

Figure 10. Electronic equivalent circuit (EEC) used in the fitting procedure of the EIS experimental data.

Table 2. Electrochemical parameters derived from impedance fitting for investigated specimens at theirrespective open circuit potentials in Ringer’s physiological solution at 37 C.

Samples Ti-6Al-4V Pure Ta Ta2N Coating

Rs (Ω·cm2) 26.25 ± 0.17 25.21 ± 0.20 34.74 ± 0.35Qp (Ω−1·cm−2·sn) (1.15 ± 0.01) × 10−5 (1.26 ± 0.01) × 10−5 (5.72 ± 0.05) × 10−6

n 0.911 ± 0.001 0.919 ± 0.001 0.941 ± 0.002Rp (Ω·cm2) (5.75 ± 0.15) × 105 (1.21 ± 0.04) × 106 (4.70 ± 0.31) × 106

Cp (µF·cm−2) 8.43 6.19 3.35τ (s) 4.85 7.49 15.75χ2 9.35 × 10−4 5.61 × 10−4 6.33 × 10−4

The Cp values for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V are 3.35, 6.19and 8.43 µF·cm−2, respectively. Higher resistance, combined with lower capacitance, endows theTa2N coating with higher insulating or dielectric property as compared to commercially pure Ta anduncoated Ti-6Al-4V. This is consistent with the result of the polarization tests. In addition, the nvalue of the Ta2N coating is greater than those of both commercially pure Ta and uncoated Ti-6Al-4V,reflecting that the passive film formed on the Ta2N coating is more homogeneous and compact thanthose formed on the two reference samples [30]. The time constant (τ) describing the charge transportprocess in the passive layer can be expressed as τ = C × R. This parameter can be directly used toevaluate the rate of relevant electrochemical process, including ionic migration within the oxide filmand charge transfer [31,32]. According to the results shown in Table 2, τ values of the tested samplesincrease in the order of uncoated Ti-6Al-4V < commercially pure Ta < the Ta2N coating. About a three

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times enhancement in time constant is achieved by implementation of a Ta2N coating on the Ti-6Al-4Vsubstrate, indicating that the Ta2N coating can effectively retard the electrochemical corrosion process.

3.6. Potentiostatic Polarization Tests

The compactness is one of the key factors that influence the stability of the passive film and canbe assessed by potentiostatic polarization analysis. Assuming the contribution of the double layercharge is negligible, the initial drop in current density should be related to the growth of the passivefilm on the electrode surface after cathodic reduction [33]. The current decreased with time accordingto the formula [34]:

i = 10−(A+klgt) (4)

where k represents the slope of the double-log plot for potentiostatic polarization, i is the current density,t is time and A is constant. k = −1 usually signifies the formation of a compact, highly protectivepassive film, while k = −0.5 indicates the formation of a porous film growing from a dissolution andprecipitation process. Figure 11 shows the double-log plots of potentiostatic current-time transientsrecorded at 0.8 V for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V in Ringer’sphysiological solution at 37 C. The values of k for the tested specimens increases in the followingsequence: uncoated Ti-6Al-4V < commercially pure Ta < the Ta2N coating, implying the increasedcompactness of the passive films formed on the tested samples. Hence, the passive film formed onTa2N coating with higher compactness exhibits a greater protective ability to hinder the penetration ofcorrosive species into the underlying film, as compared to the two reference samples.

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3.6. Potentiostatic Polarization Tests

The compactness is one of the key factors that influence the stability of the passive film and can be assessed by potentiostatic polarization analysis. Assuming the contribution of the double layer charge is negligible, the initial drop in current density should be related to the growth of the passive film on the electrode surface after cathodic reduction [33]. The current decreased with time according to the formula [34]: = 10 ( ) (4)

where k represents the slope of the double-log plot for potentiostatic polarization, i is the current density, t is time and A is constant. k = −1 usually signifies the formation of a compact, highly protective passive film, while k = −0.5 indicates the formation of a porous film growing from a dissolution and precipitation process. Figure 11 shows the double-log plots of potentiostatic current-time transients recorded at 0.8 V for the Ta2N coating, commercially pure Ta and uncoated Ti-6Al-4V in Ringer’s physiological solution at 37 °C. The values of k for the tested specimens increases in the following sequence: uncoated Ti-6Al-4V < commercially pure Ta < the Ta2N coating, implying the increased compactness of the passive films formed on the tested samples. Hence, the passive film formed on Ta2N coating with higher compactness exhibits a greater protective ability to hinder the penetration of corrosive species into the underlying film, as compared to the two reference samples.

Figure 11. Double-log plots of current-time for uncoated Ti-6Al-4V, commercially pure Ta and the Ta2N coating potentiostatically polarizated at 0.8 V in Ringer’s physiological solution.

3.7. XPS Analysis of Passive Film Composition

In order to gain insight into the change in the composition and chemical state of the elements of the passive films at different applied potentials, XPS analysis was performed ex situ on the Ta2N coating after potentiostatic polarization at 0.2 V and 0.8 V for 60 min in Ringer’s physiological solution at 37 °C (Figure 12). As shown in Figure 12a, the XPS survey spectra of the two passive films grown on the Ta2N coating exhibit peaks from Ta, O, N and C. The C1s peak possibly arises from a contaminant hydrocarbon layer covering the outermost surface of the specimen. Figure 12b,c show high-resolution XPS spectra for Ta 4f and Ta 4p/N 1s core levels regions obtained from the two passive films formed on the Ta2N coating, respectively. After deconvolution, the Ta 4f spectrum (Figure 12b) recorded from the passive film formed at 0.2 V presents two sets of doublets of Ta 4f7/2 and Ta 4f5/2. One doublet at the low-energy-side with a binding energy of Ta 4f7/2 at 22.3 eV can be assigned to Ta2N [35], while another high-energy-side doublet with a binding energy of Ta 4f7/2 positioned at 25.9 eV can be attributed to TaOxNy [36]. Note that the detection of Ta2N may possibly originate from the Ta2N coating underneath the passive film. In the case of the passive film formed at 0.8 V, the Ta 4f core level shows one doublet with a binding energy of Ta 4f7/2 located at 26.4 eV, which corresponds to the binding energy of Ta in stoichiometric Ta2O5 [37]. In Figure 12c,

Figure 11. Double-log plots of current-time for uncoated Ti-6Al-4V, commercially pure Ta and the Ta2Ncoating potentiostatically polarizated at 0.8 V in Ringer’s physiological solution.

3.7. XPS Analysis of Passive Film Composition

In order to gain insight into the change in the composition and chemical state of the elementsof the passive films at different applied potentials, XPS analysis was performed ex situ on the Ta2Ncoating after potentiostatic polarization at 0.2 V and 0.8 V for 60 min in Ringer’s physiological solutionat 37 C (Figure 12). As shown in Figure 12a, the XPS survey spectra of the two passive films grown onthe Ta2N coating exhibit peaks from Ta, O, N and C. The C1s peak possibly arises from a contaminanthydrocarbon layer covering the outermost surface of the specimen. Figure 12b,c show high-resolutionXPS spectra for Ta 4f and Ta 4p/N 1s core levels regions obtained from the two passive films formed onthe Ta2N coating, respectively. After deconvolution, the Ta 4f spectrum (Figure 12b) recorded from thepassive film formed at 0.2 V presents two sets of doublets of Ta 4f7/2 and Ta 4f5/2. One doublet at thelow-energy-side with a binding energy of Ta 4f7/2 at 22.3 eV can be assigned to Ta2N [35], while anotherhigh-energy-side doublet with a binding energy of Ta 4f7/2 positioned at 25.9 eV can be attributed to

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TaOxNy [36]. Note that the detection of Ta2N may possibly originate from the Ta2N coating underneaththe passive film. In the case of the passive film formed at 0.8 V, the Ta 4f core level shows one doubletwith a binding energy of Ta 4f7/2 located at 26.4 eV, which corresponds to the binding energy of Ta instoichiometric Ta2O5 [37]. In Figure 12c, XPS spectra for the Ta 4p3/2/N 1s region obtained from thepassive film formed at 0.2 V can be deconvoluted into four peak components. Two peaks located atbinding energies of 404.2 eV and 396.6 eV can be assigned to the Ta 4p3/2 peak and the N 1s peak ofTaOxNy [36]. Another N 1s peak at a binding energy of 397.2 eV is characteristic of the Ta-N bond inTa2N [36]. The component with a binding energy of 400.1 eV corresponds to the N 1s binding energyof N–H in NH4

+ [38]. Compared to that for TaOxNy, the Ta 4p3/2 peak recorded from the passive filmformed at 0.8 V is shifted to a higher binding energy by 0.8 eV, which is typical of the Ta chemical statesin stoichiometric Ta2O5 [37]. The shift in binding energies for Ta 4p3/2 is attributed to the covalencybetween Ta and O being less than between Ta and N, because nitrogen is less electronegative thanoxygen [39]. In addition, the peak at a binding energy of 400.1 eV was also found on the passive filmformed at 0.8 V. According to the principal constituent of the passive films formed on the Ta2N coating,two possible hydrolysis reactions can be proposed to describe the chemical interaction of the Ta2Ncoating with Ringer’s physiological solution at different applied potentials:

Ta2N + H2O→ TaOxNy + NH4+ + OH− (5)

TaOxNy + H2O→ Ta2O5 + NH4+ + OH− (6)

Firstly, at a low applied potential of 0.2 V, the oxidation of the Ta2N coating leads to the formationof a tantalum oxynitride (TaOxNy) passive film on the coating surface by Reaction (5), and subsequentlythe tantalum oxynitride (TaOxNy) can be chemically converted by oxidation (Reaction (6)) into Ta2O5

at a higher applied potential of 0.8 V.

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XPS spectra for the Ta 4p3/2/N 1s region obtained from the passive film formed at 0.2 V can be deconvoluted into four peak components. Two peaks located at binding energies of 404.2 eV and 396.6 eV can be assigned to the Ta 4p3/2 peak and the N 1s peak of TaOxNy [36]. Another N 1s peak at a binding energy of 397.2 eV is characteristic of the Ta-N bond in Ta2N [36]. The component with a binding energy of 400.1 eV corresponds to the N 1s binding energy of N–H in NH4+ [38]. Compared to that for TaOxNy, the Ta 4p3/2 peak recorded from the passive film formed at 0.8 V is shifted to a higher binding energy by 0.8 eV, which is typical of the Ta chemical states in stoichiometric Ta2O5 [37]. The shift in binding energies for Ta 4p3/2 is attributed to the covalency between Ta and O being less than between Ta and N, because nitrogen is less electronegative than oxygen [39]. In addition, the peak at a binding energy of 400.1 eV was also found on the passive film formed at 0.8 V. According to the principal constituent of the passive films formed on the Ta2N coating, two possible hydrolysis reactions can be proposed to describe the chemical interaction of the Ta2N coating with Ringer’s physiological solution at different applied potentials: Ta N + H O → TaO N + NH + OH (5) TaO N + H O → Ta O + NH + OH (6)

Firstly, at a low applied potential of 0.2 V, the oxidation of the Ta2N coating leads to the formation of a tantalum oxynitride (TaOxNy) passive film on the coating surface by Reaction (5), and subsequently the tantalum oxynitride (TaOxNy) can be chemically converted by oxidation (Reaction (6)) into Ta2O5 at a higher applied potential of 0.8 V.

Figure 12. Cont.

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Figure 12. (a) XPS survey spectra and (b,c) high-resolution XPS spectra for Ta 4f and Ta 4p/N 1s for the passive films formed on the Ta2N coating after potentiostatic polarization at 0.2 V and 0.8 V for 60 min in Ringer’s physiological solution.

3.8. Mott-Schottky Analysis and Point Defect Model

In general, the protective films formed on passivated materials, regarded as defective oxide layers, are known to exhibit the semiconducting properties, which have been correlated with their electrochemical behavior [40]. Mott-Schottky analysis, through measuring of the electrode capacitance as a function of applied potential, has been proven to be a powerful tool for providing precise information on the electronic properties of the passive films [41]. According to the Mott-Schottky theory [42], under the depletion conditions, the linear relation between the reciprocal of the square of the space charge capacitance (Csc) and the applied potential (E) can be expressed as: 1 = 1 = 2 − − (7)

where εr is the dielectric constant of the passive film (~60 [41] for TiO2 and ~25 [43] for Ta2O5), ε0 is the vacuum permittivity (8.854 × 10−14 F·cm−1), q is the elementary charge (+e for electrons and –e for holes), Nq is the density of charge carriers (Nd for donors and Na for acceptors), E is the applied potential, Efb is the flat band potential, k is the Boltzmann constant (1.38 × 10−23 J·K−1), and T is absolute temperature (310 K). The term kT/q can be neglected since it is only ~27 mV at 310 K. The measured interfacial capacitance, C, is obtained from C = −1/ωZ", where ω = 2πf is the angular frequency and Z" is the imaginary part of the impedance. The Mott-Schottky analysis is based on the assumption that the contribution of the Helmholtz double-layer capacitance (CH) can be neglected and the capacitance of the passive films is mainly dominated by the CSC. In our case, this assumption is valid since the measured capacitances on the tested specimens (as shown in Table 2) are far less than the values of CH reported in the literature (about 20~50 μF·cm−2 [44,45]).

Figure 13 shows the reciprocal of the square of the Csc as a function of the applied potential for the passive films formed on the three tested specimens after being polarized at different film formation potentials (Ef) for 60 min in Ringer’s physiological solution at 37 °C. It can be seen that the Mott-Schottky plots can be decomposed into two linear regions over the entire potential range, and the slopes of two linear regions in each plot are always positive, which is characteristics of n-type semiconductors behavior. Such two linear regions located at low and high potentials have also been reported for other metal oxides in different media [46–48]. This phenomenon was attributed to a transition of passive film character from semiconducting behavior to dielectric behavior, as discussed in our recent work [49]. Furthermore, it can be seen from Figure 13 that the magnitude of Csc−2 and the slopes of Csc−2 vs. E increase with increasing the Ef, denoting that the donor densities (Nd) decrease with an increase in Ef. These accords with theoretical prediction based on the point defect model (PDM) [50]. Schmidt et al. [51] suggested that the higher Ef was favorable for the formation of a highly ordered and denser passive film, accompanied by a reduction in carrier density. As shown in Figure 13(b), it is important to note that at the Ef values of 0.6, 0.8 and 1.0 V, the negative slopes of

Figure 12. (a) XPS survey spectra and (b,c) high-resolution XPS spectra for Ta 4f and Ta 4p/N 1s forthe passive films formed on the Ta2N coating after potentiostatic polarization at 0.2 V and 0.8 V for60 min in Ringer’s physiological solution.

3.8. Mott-Schottky Analysis and Point Defect Model

In general, the protective films formed on passivated materials, regarded as defective oxidelayers, are known to exhibit the semiconducting properties, which have been correlated with theirelectrochemical behavior [40]. Mott-Schottky analysis, through measuring of the electrode capacitanceas a function of applied potential, has been proven to be a powerful tool for providing preciseinformation on the electronic properties of the passive films [41]. According to the Mott-Schottkytheory [42], under the depletion conditions, the linear relation between the reciprocal of the square ofthe space charge capacitance (Csc) and the applied potential (E) can be expressed as:

1Csc2 =

1C2 =

2εrε0qNq

(E− E f b −

kTq

)(7)

where εr is the dielectric constant of the passive film (~60 [41] for TiO2 and ~25 [43] for Ta2O5),ε0 is the vacuum permittivity (8.854 × 10−14 F·cm−1), q is the elementary charge (+e for electronsand –e for holes), Nq is the density of charge carriers (Nd for donors and Na for acceptors), E is theapplied potential, Efb is the flat band potential, k is the Boltzmann constant (1.38 × 10−23 J·K−1), andT is absolute temperature (310 K). The term kT/q can be neglected since it is only ~27 mV at 310 K.The measured interfacial capacitance, C, is obtained from C = −1/ωZ", where ω = 2πf is the angularfrequency and Z" is the imaginary part of the impedance. The Mott-Schottky analysis is based on theassumption that the contribution of the Helmholtz double-layer capacitance (CH) can be neglected andthe capacitance of the passive films is mainly dominated by the CSC. In our case, this assumption isvalid since the measured capacitances on the tested specimens (as shown in Table 2) are far less thanthe values of CH reported in the literature (about 20~50 µF·cm−2 [44,45]).

Figure 13 shows the reciprocal of the square of the Csc as a function of the applied potentialfor the passive films formed on the three tested specimens after being polarized at different filmformation potentials (Ef) for 60 min in Ringer’s physiological solution at 37 C. It can be seen thatthe Mott-Schottky plots can be decomposed into two linear regions over the entire potential range,and the slopes of two linear regions in each plot are always positive, which is characteristics ofn-type semiconductors behavior. Such two linear regions located at low and high potentials have alsobeen reported for other metal oxides in different media [46–48]. This phenomenon was attributed toa transition of passive film character from semiconducting behavior to dielectric behavior, as discussedin our recent work [49]. Furthermore, it can be seen from Figure 13 that the magnitude of Csc

−2

and the slopes of Csc−2 vs. E increase with increasing the Ef, denoting that the donor densities (Nd)

decrease with an increase in Ef. These accords with theoretical prediction based on the point defectmodel (PDM) [50]. Schmidt et al. [51] suggested that the higher Ef was favorable for the formation of

Materials 2016, 9, 772 15 of 21

a highly ordered and denser passive film, accompanied by a reduction in carrier density. As shownin Figure 13b, it is important to note that at the Ef values of 0.6, 0.8 and 1.0 V, the negative slopes ofthe straight lines are observed for commercially pure Ta at potentials above 0.3 V, which is attributedto the formation of an inversion layer derived from an increase in hole concentration in the valenceband [52].

Materials 2016, 9, 772 15 of 21

the straight lines are observed for commercially pure Ta at potentials above 0.3 V, which is attributed to the formation of an inversion layer derived from an increase in hole concentration in the valence band [52].

Figure 13. Mott-Schottky plots of the passive films formed at different potentials (0.4, 0.6, 0.8, 1.0 V) on (a) uncoated Ti-6Al-4V; (b) commercially pure Ta and (c) the Ta2N coating in Ringer’s physiological solution.

According to Equation (7), the calculated Nd and Efb values for the passive films formed on the three tested samples at different potentials in Ringer’s physiological solution at 37 °C are summarized in Table 3. As shown in Table 3, with increasing the Ef, the Nd decreases and the Efb moves slightly to more positive potential for all of the tested samples. At a given Ef, values of Nd obtained from the passive film on the Ta2N coating are in the range of 1018–1019 cm−3, which are substantially lower than those for the passive films formed on both uncoated Ti-6Al-4V and commercially pure Ta. It is known that the larger Nd represents the higher conductivity of the passive film, which will result in an increase in the passive current density and reduce the protective effect of the passive film. From the electronic standpoint, lower Nd in the passive film on the Ta2N coating restrains the transfer of the electron and impedes the electrochemical reaction in the passive film. This results account for the different passive current density of the three tested

Figure 13. Mott-Schottky plots of the passive films formed at different potentials (0.4, 0.6, 0.8,1.0 V) on (a) uncoated Ti-6Al-4V; (b) commercially pure Ta and (c) the Ta2N coating in Ringer’sphysiological solution.

According to Equation (7), the calculated Nd and Efb values for the passive films formed on thethree tested samples at different potentials in Ringer’s physiological solution at 37 C are summarizedin Table 3. As shown in Table 3, with increasing the Ef, the Nd decreases and the Efb moves slightlyto more positive potential for all of the tested samples. At a given Ef, values of Nd obtained fromthe passive film on the Ta2N coating are in the range of 1018–1019 cm−3, which are substantiallylower than those for the passive films formed on both uncoated Ti-6Al-4V and commercially pureTa. It is known that the larger Nd represents the higher conductivity of the passive film, which will

Materials 2016, 9, 772 16 of 21

result in an increase in the passive current density and reduce the protective effect of the passivefilm. From the electronic standpoint, lower Nd in the passive film on the Ta2N coating restrains thetransfer of the electron and impedes the electrochemical reaction in the passive film. This resultsaccount for the different passive current density of the three tested samples shown in Figure 8 [53,54].Moreover, the presence of aggressive chloride ions (Cl−) in Ringer’s physiological solution will exerta detrimental effect on the stability of the passive film. According to the point defect model (PDM)proposed by Macdonald [50], chloride ions can be absorbed into oxygen vacancies at the passivefilm/solution interface. The absorption of chloride ions into the oxygen vacancies rise the local cationvacancy concentration and then enhances the electromigration-dominated flux of cation vacanciesfrom the barrier layer/solution interface to the metal or coating/barrier layer interface, where they areannihilated by an oxidative injection of cation from the metal or coating into the film. If the annihilationreaction cannot accommodate the enhanced flux of cation vacancies, the excess vacancies will condenseto form voids, causing a collapse of the film. Hence, compared with those on the uncoated Ti-6Al-4Vand commercially pure Ta, the decreased oxygen vacancies of the passive film on the Ta2N coating canlower the cation vacancy concentration at the metal or coating/barrier layer interface and enhance thestability of the passive film.

Table 3. Summary of parameters derived from capacitance measurements and PDM analysis for thepassive films formed on the investigated specimens in Ringer’s physiological solution.

Samples Ti-6Al-4V Pure Ta Ta2N Coating

0.4 VNd (×1019 cm−3) 11.13 17.26 1.48

Efb (V) −1.15 −1.66 −1.50δsc (nm) 9.61 5.74 18.85

0.6 VNd (×1019 cm−3) 5.92 13.75 1.10

Efb (V) −0.90 −1.57 −1.49δsc (nm) 12.96 6.60 22.93

0.8 VNd (×1019 cm−3) 3.42 11.53 0.80

Efb (V) −0.80 −1.53 −1.47δsc (nm) 17.61 7.47 28.02

1.0 VNd (×1019 cm−3) 2.20 10.81 0.62

Efb (V) −0.78 −1.52 −1.48δsc (nm) 23.16 8.02 33.27

ω2 (×1019 cm−3) 1.08 9.62 0.15iss (×10−6A·cm−2) 9.50 3.49 0.10Do (×10−16 cm2/s) 27.33 1.13 1.94

The thickness of the space-charge layer (δsc) in a semiconductor is an important parameter thatcontrols the movement of electrons. The space charge layer thickness (δsc) for an n-type semiconductorcan be calculated from the following equation [55]:

δsc = [2εrε0

eNd(E− E f b −

kTe)]

1/2(8)

As shown in Table 3, at a given Ef, the Ta2N coating has the largest δsc, while commercially pureTa has the lowest. Because the space charge layer thickness scales with the thickness of the oxide layer,thicker passive films are less susceptible to breakdown and pitting, and provide higher protectionagainst corrosion [56]. Thereby, it is reasonable to conclude that, under identical passivation conditions,the thickness of the passive film grown on the tested specimens increases in the order of commerciallypure Ta < uncoated Ti-6Al-4V < Ta2N coating.

The flux of the point defects is essential to the growth and breakdown of the passive film.The diffusivity (Do) qualitatively describes the transport of the point defects in the passive film from

Materials 2016, 9, 772 17 of 21

a microscopic perspective based on the PDM. It is reported that the field strength of the passive filmcan reach a magnitude of approximately 106 V/cm and, under a high field strength condition, thediffusivity (Do) can be calculated by Equation (9) [46,57]:

Do =zαiss

eω2exp(zαεF/RT)(9)

where z is the charge number of the mobile point defects in the passive film (z = 2 is used in this work),α is the half-jump distance of migrating point defects, assuming α = 0.25 nm [57], iss is the steadystate passive current density, which can be determined from the polarization curves, e is the charge ofan electron (1.6 × 10−19 C), ω2 is a unknown constant that can be acquired by exponentially fittingthe Nd-Ef plot (shown in Figure 14), ε is the mean field strength of the passive film (approximately3.0 × 106 V/cm [58]), F is the Faraday constant (96.485 C/mol), R is the gas constant (8.314 J/K·mol),and T is the temperature in Kelvin (310 K).

Materials 2016, 9, 772 17 of 21

= exp( / ) (9)

where z is the charge number of the mobile point defects in the passive film (z = 2 is used in this work), α is the half-jump distance of migrating point defects, assuming α = 0.25 nm [57], iss is the steady state passive current density, which can be determined from the polarization curves, e is the charge of an electron (1.6 × 10−19 C), ω2 is a unknown constant that can be acquired by exponentially fitting the Nd-Ef plot (shown in Figure 14), is the mean field strength of the passive film (approximately 3.0 × 106 V/cm [58]), F is the Faraday constant (96.485 C/mol), R is the gas constant (8.314 J/K·mol), and T is the temperature in Kelvin (310 K).

Figure 14. Donor density (Nd) in the passive films formed on (a) uncoated Ti-6Al-4V; (b) commercially pure Ta and (c) the Ta2N coating in Ringer’s physiological solution as a function of film formation potential (Ef). The solid lines are the exponential fit of Nd on Ef.

In order to obtain the value of ω2, the dependence of Nd on Ef was fitted using a first-order exponential decay function [46,50,57]:

Figure 14. Donor density (Nd) in the passive films formed on (a) uncoated Ti-6Al-4V; (b) commerciallypure Ta and (c) the Ta2N coating in Ringer’s physiological solution as a function of film formationpotential (Ef). The solid lines are the exponential fit of Nd on Ef.

Materials 2016, 9, 772 18 of 21

In order to obtain the value of ω2, the dependence of Nd on Ef was fitted using a first-orderexponential decay function [46,50,57]:

Nd = ω1exp(−bE f

)+ ω2 (10)

The Nd of the passive films formed on the tested specimens under different Ef and thecorresponding exponential fitted results are shown in Figure 14. The values of ω2 for the passivefilms formed on uncoated Ti-6Al-4V, commercially pure Ta and the Ta2N coating are 1.08 × 1019 cm−3,9.62 × 1019 cm−3 and 1.50 × 1018 cm−3, respectively. Then, substituting the variables iss, ω2, z, e, ε, F,R, T, and α into Equation (9) to calculate the diffusivity of the point defects in the passive films, whichare listed in Table 3. The calculated Do value for the Ta2N coating (1.94× 10−16 cm2/s) is comparable tothat for commercially pure Ta (1.13× 10−16 cm2/s), both of which are one order of magnitude less thanthat for uncoated Ti-6Al-4V (2.73 × 10−15 cm2/s). As a result, it can be concluded that the lower Nd,combined with the lower Do for the Ta2N coating, impedes the electrochemical reaction from occurringon its passive film and improves the stability of the passive film. This enhances the protective abilityof the Ta2N coating. Moreover, it can be noted here that during the anodic polarization of the Ta2Ncoating, the formation of ammonium ions consumes protons that raises the local pH [59], reducingthe acidifying effect of metal dissolution. The NH4

+ ions may possibly undergo further reaction tothe formation of stable NOx

− species, such as NO2− or NO3

−, which can further improve resistanceto pitting through repelling the adsorption of chloride ions on the film/electrolyte interface. By theway, compared with uncoated Ti-6Al-4V, commercially pure Ta has a higher donor density and thinnerpassive film, yet it shows a higher corrosion resistance in Ringer’s physiological solution, mainlyoriginating from its lower Do.

4. Conclusions

In this study, a Ta2N nanoceramic coating was fabricated onto Ti-6Al-4V substrate in an Arand N2 gas mixture using a double glow discharge plasma process. The as-deposited coatingconsists of fine nanocrystals with an average grain size of 12.8 nm. The coating enhances thesurface hardness of Ti-6Al-4V and exhibits good contact damage tolerance and sufficient adhesionstrength to meet requirements for long-term use in the human body under load-bearing conditions.The corrosion resistance of the Ta2N coating was investigated by various electrochemical analyticaltechniques in Ringer’s physiological solution at 37 C, and was compared to commercially pure Ta anduncoated Ti-6Al-4V. Potentiodynamic polarization showed that the Ta2N coating could be passivatedspontaneously in Ringer’s physiological solution and showed a more positive Ecorr and lower icorr

than both commercially pure Ta and uncoated Ti-6Al-4V. EIS results indicated that the resistance value(Rp) of the Ta2N coating was three times greater than that of commercially pure Ta, and was one orderof magnitude larger than that of uncoated Ti-6Al-4V. The Mott-Schottky analysis indicated that thepassive film formed on the Ta2N coating are in the range of 1018–1019 cm−3, which are substantiallylower than those for the passive films formed on both commercially pure Ta and uncoated Ti-6Al-4V.In light of its high electrochemical stability combined with good mechanical properties, the Ta2Ncoating has potential for the surface protection of metallic orthopedic devices.

Acknowledgments: The authors acknowledge the financial support from the General Program of the NationalNature Science Foundation of China under Grant No. 51374130 and No. 51675267, and Key Program of theNational Natural Science Foundation of China under Grant No. 51635004. This work is also supported byFunding of Jiangsu Innovation Program for Graduate Education, the Fundamental Research Funds for the CentralUniversities (CXLX13-151). A Project Funded by the Priority Academic Program Development of Jiangsu HigherEducation Institutions.

Author Contributions: Jian Cheng and Jiang Xu conceived and designed the experiments; Jian Cheng andLin Lin Liu performed the experiments; Lin Lin Liu and Jiang Xu analyzed the data; Shuyun Jiang contributedreagents/materials/analysis tools; Jian Cheng and Jiang Xu wrote the paper.

Conflicts of Interest: The authors declare no conflicts of interest.

Materials 2016, 9, 772 19 of 21

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